It is known that the body waves, generally in the acoustic or ultrasonic range generated in body parts of a machine, especially a rotating machine or a machine having a rotating element, can be utilized as a signal of a defect in the operation.
It is known, for example, to utilize transducers or pickups responsive to the wave energy in a machine body to measure a first body-wave parameter in a high-frequency range and a second body-wave parameter in a low-frequency range of the frequency spectrum generated upon machine operation and to compare the signals representing the picked up wave components in a comparator.
The comparator may be or can include a computer.
A process of this type is effectively used to monitor vibration in machine parts and, for example, for monitoring the conditions of bearings, transmission and turbine sets.
When the monitoring shows a change from a prior condition, this can represent a failure of the part or some other part associated with the machine.
The change in the oscillatory state or condition to which the device must respond, however, requires comparatively little energy so that the defect may be ascertained even upon startup.
The body-wave parameter which is measured directly or indirectly can be a function of the effective value of the acceleration, can be the first integral of this acceleration, i.e. the oscillation speed or velocity and/or the second integral of this acceleration or the first integral of the oscillation velocity, i.e. the displacement, lateral deflection or path deviation.
The measurements can be taken by appropriate transducers and subjected, with conventional frequency analyzers, to analysis, transformation and various forms of combination.
In one conventional technique (see Olhydraulik und Pneumatik, 1981, P. 568-573), the first body-wave parameter and the second body-wave parameter are measured in the same range of the frequency spectrum and at different times. The first body-wave parameter is measured when the machine and therefore the part to be monitored are newly in operation so that the first body-wave parameter can be constituted a reference value or can generate a reference value which is stored for later use. At a later time, this reference value is compared to the second body-wave parameter or a signal generated thereby or associated therewith and a deviation of the measured value from the reference value can be ascertained as an indication of failure.
It is also possible to utilize two body-wave parameters for this purpose, the first body-wave or reference parameter being measured at one frequency range of the body-wave spectrum while the second is measured at another frequency range. One of these frequency ranges may be a comparatively high frequency while the other frequency range may be a comparatively low frequency.
Since the second measurement of the body-wave parameter is not simultaneous or concurrent with the first or reference parameter, the measurement is not coherent. Coherency, as this term is used herein, refers to the ability to generate correlation relationships by spectral analysis, i.e. correlation between the phases of sperimposed waves generally with random variables arising from stochastic processes. Coherency with respect to waves is related to the tendency to generate interference interactions.
While these earlier techniques have proved to be satisfactory for many purposes, they have the important drawback that the measurements are only pertinent to a specific machine under specific conditions. Neither reference value measurement nor the second measurement at a later point in time can be utilized from one machine to the next even with mass produced or serially built machines which are of similar or identical construction. All of the values and calculations, therefore, must be done utilizng pickups on the respective machines at startup and operation.
The measurements, moreover, do not automatically allow anticipation of a defect state. This is because the frequency spectra are strongly dependent on the operating conditions and hence monitoring under conditions which vary with time is difficult, if not impossible, with conventional techniques.
For example, in a rotating machine which must operate at different speeds not only must measurement be taken to serve as a reference value in each machine of the class, but for each of the range of speeds at which the machine may operate if an accurate evaluation of the development of a defect state is to be ensured.